Adam Warner won the 3k race at the London Region Open Water Championships at Surrey Docks.
Warner missed last year's competition due to Maccabiah commitments and he used the race as preparation for next weekend British Gas National Open Water Championships in Sheffield.
The six-lap, 500m circuit saw swimmers compete in perfect conditions, with the water temperature was around 23 degrees.
The race started from a dive on the pontoon. Warner was one of the first into the water and made his way to the front of the pack and was battling with a GB swimmer going to the European Championships in a few weeks.
The two swimmers moved clear of the field, with the lead changing hands several times before Warner made a decisive move to the finish line.
Warner said: "It was an amazing feeling to beat a GB swimmer, as all the work was done by myself, with no-one to help me out. But the main thing is, it was great preparation for next weekend, where I'll be competing against the fastest open water swimmers in Great Britain."